Witnessed a JunitTest_chart2_unoapi failure where (deep within Idle::Timeout)
chart::impl::ModelState::update is apparently called with a null xModel.  All
the code called from there appears to be careful to handle a null xModel, except
for chart::TitleHelper::getTitle, which---presumably by accident---stopped doing
so with 401f01ca "use ChartModel instead of
XModel in a few places."

Change-Id: I5788b5e41a6d68fe193cb214d736ee54c4c581d7